Along with the trend of emphasizing the crashworthiness and fuel efficiency of automobiles, the use of high tensile steel for manufacturing automotive chassis has increased to satisfy the demand for lightweight, high-strength automobiles. In addition, the use of high-strength steel for manufacturing automotive outer panels has also increased. At present, 340-MPa grade bake hardening steel sheets are used for manufacturing most automotive outer panels, and 490-MPa grade steel sheets are used for manufacturing some automotive outer panels. Also, the use of 590-MPa grade steel sheets is also expected.
Such high-strength steel sheets used for manufacturing automotive outer panels have lightweight and dent-resistant features but have poor formability due to the high-strength features thereof. In addition, if high-strength steel sheets having a low hole expansion ratio (HER) are used, cracks may be formed in edge regions of machined portions of the high-strength steel sheets. Therefore, automobile manufacturers have recently required high-strength steel sheets having an improved HER for automotive outer panels to compensate for poor workability of the high-strength steel sheets by the high HER. In addition, steel sheets for automotive outer panels should have a high degree of surface quality. However, since oxidizable hardening elements such as silicon (Si) and manganese (Mn) are included in such steel sheets to increase the strength of the steel sheets, it is difficult to obtain a high degree of surface quality after plating the steel sheets.
Moreover, steel sheets for automobiles should have a high degree of corrosion resistance. Thus, hot-dip galvanized steel sheets having a high degree of corrosion resistance have long been used for manufacturing automobiles. Since hot-dip galvanized steel sheets are produced using continuous hot-dip galvanizing equipment in which a recrystallization annealing process and a galvanizing process are performed in the same production line, the hot-dip galvanized steel sheets having a high degree of corrosion resistance may be manufactured with low costs. In addition, hot-dip galvannealed steel sheets obtained by heat-treating hot-dip galvanized steel sheets have a high degree of corrosion resistance in addition to a high degree of weldability or formability, and thus they are widely used.
As described above, the development of high tensile cold-rolled steel sheets having a high degree of formability has been required to manufacture automotive outer panels having lightweight and workable features, and along with this, the development of high tensile hot-dip galvanized steel sheets having high degrees of corrosion resistance, weldability, and formability has also been required.
A technique for improving the workability of high tensile steel sheets is disclosed in Patent Document 1 (Japanese Patent Application Laid-open Publication NO.: 2005-264176). Patent Document 1 discloses a multi-phase steel sheet having martensite as a main phase, and a method of manufacturing a high tensile steel sheet by dispersing fine copper (Cu) precipitates having a particle diameter of 1 nm to 100 nm in the microstructure of the steel sheet to improve the workability of the steel sheet. According to the technique disclosed in Patent Document 1, copper (Cu) has to be added in a large amount of 2% to 5% in order to precipitate fine copper (Cu) particles. However, copper (Cu) may cause red brittleness and incurs high manufacturing costs.
Patent Document 2 (Japanese Patent Application Laid-open Publication No.: 2004-292891) discloses a multi-phase steel sheet including ferrite as a main phase, retained austenite as a secondary phase, and bainite and martensite as low-temperature transformation phases, and a method of improving the ductility and stretch-flangeability of the multi-phase steel sheet. According to the technique disclosed in Patent Document 2, however, since large amounts of silicon (Si) and aluminum (Al) are added to ensure the formation of retained austenite, it is difficult to ensure plating quality, and the surface quality of steel is not guaranteed during a steel making process and a continuous casting process. In addition, the initial yield strength (YS) of the multi-phase steel sheet of Patent Document 2 is high because of transformation-induced plasticity, and thus the yield ratio of the multi-phase steel is high.
Patent Document 3 (Korean Patent Application Laid-open Publication No.: 2002-0073564) discloses a high tensile hot-dip galvanized steel sheet having satisfactory workability and including soft ferrite and hard martensite in the microstructure thereof, and a method of manufacturing the steel sheet while improving the elongation and r value (Lankford value) of the steel sheet. However, since the technique disclosed in Patent Document 3 requires the addition of a large amount of silicon (Si), it is difficult to obtain a high degree of galvanizing quality. In addition, since the technique requires the addition of large amounts of titanium (Ti) and molybdenum (Mo), manufacturing costs are high.